It is known to produce surface acoustic wave devices that use the propagation of waves on the surface of a piezoelectric substrate. In the case of what are called Rayleigh waves, these are generated and received by interdigitated comb transducers composed of interlaced electrodes between which a potential difference is imposed. These devices have two main drawbacks.
Firstly, in order for the surface waves to propagate correctly on the surface of the substrate, this surface must remain free. This condition is obtained by encapsulation technologies for obtaining a cavity.
Secondly, the pitch of the electrodes making up the interdigitated combs is often small, of the order of a few hundred nanometers. In addition, conducting particles of very small dimensions present inside the package may short-circuit a transducer and disturb the normal operation of the device. To alleviate this drawback, it is necessary either to make the packages for the components hermetically sealed, or to deposit a thin layer of insulating dielectric material on the transducers. This operation, called passivation, makes it possible to eliminate sensitivity to conducting particles. However, passivation does not make it possible to dispense with the encapsulation operation, which is an operation expensive to carry out.
The use of interface acoustic wave devices allows the various problems associated with encapsulation to be solved. In this case, use is no longer made of the propagation of the acoustic waves on the surface of the substrate, but at the interface between two substrates instead. Of course, this device makes it possible to obtain a passivated component no longer requiring a cavity to be produced. Moreover, the package may be completely eliminated.
In 1924, Stoneley demonstrated the possibility of guiding an acoustic wave at the interface between two materials [Proc. Roy. Soc. London A 106, 416]. These waves were regarded as being polarized in the sagittal plane.
In 1971, Maerfeld and Tournois demonstrated the existence of horizontal shear waves propagating at the surface between two materials. The piezoelectric case was considered [C. Maerfeld and P. Tournois, Appl. Phys. Lett. 19, 117, 1971]. The first use of this type of wave for acoustic components is disclosed in patent FR 2 145 750. The invention described uses the propagation of pure shear waves at the interface between two materials, at least one of which is piezoelectric. The case in which the two materials are identical is considered. However, the above patent makes no mention of transducers placed at the interface between the two materials.
In 1983, the propagation of waves at the interface between a piezoelectric material and an isotropic material, for the purpose of producing packageless SAW (surface acoustic wave) devices is described, which implicitly assumes that the transducers are placed at the interface. The coupling coefficient was also studied [Shimitzu et al., “Stoneley waves propagating along an interface between piezoelectric material and isotropic material”, 1983 IEEE U.S. Proc. pp 373–375].
More recently, in 1998, a different combination of materials was examined for the purpose of filtering [M. Yamaguchi, T. Yamashita, K. Hashimoto and T. Omori, “Highly piezoelectric waves in Si/SiO2/LiNbO3 and Si/SiO2/LiNbO3 structures” (unpublished)].
Finally, in 1999, patent FR 2 799 906 describes filters using transducers at the interface between two identical piezoelectric materials.
In general, an interface acoustic wave device consists of two substrates denoted by S1 and S2, at least one of which is piezoelectric, and of an interface region I lying between these two substrates, as indicated in FIG. 1. In the general case, the interface region I is a structure that comprises at least the electro-acoustic transducers E. Electrical interconnections coupled to said devices allow signals to be emitted and transmitted.
The interface waves can be used to produce passive components. In general, any type of device obtained using waves propagating on the surface of a crystal can be produced using interface waves. In particular, it is possible to reflect the interface waves using arrays of metal electrodes with a period equal to a half-wavelength placed at the interface. Thus, firstly a resonator is produced, by placing an interdigitated transducer between two reflector arrays, and secondly a filter, by coupling resonators together via electrical or acoustic means. The directivity of a transducer is improved by interspersing reflectors therein. All applications of surface wave components are therefore accessible, especially delay lines, band filters, resonators and dispersive filters. Applications of these components as measurement sensors are also possible.
Phase code devices have specific transducers characterized by a distribution of the electrodes such that it is known how to associate one particular code with a given phase code component. Remotely interrogable devices using radio waves employ this principle. The operation is as follows: a phase code is used at emission of the wave, and the wave is picked up by an antenna connected to the input of the phase-code component; conventionally, the transducer converts the signal into a mechanical wave. Said wave propagates as far as the output transducer where it is then reconverted into an electrical signal and re-emitted. The received signal is analyzed and the component that has received and transformed the signal is thus identified.
Interface wave devices can be used as remotely interrogable devices, especially for measurement applications, such as for the measurement of pressure, temperature or acceleration.
The choice of structures for the interface wave components may vary very greatly. In particular, the following combinations may be mentioned:                the substrate S1 is piezoelectric and S2 is not. In this case, S2 is chosen according to its mechanical properties so as to make it easier to produce the components. For example, if S1 is made of lithium niobate or lithium tantalate, S2 will preferably be made of fused silica or single-crystal silicon;        the two substrates are both piezoelectric, but of different nature. For example, the following combinations may be mentioned:                    quartz/lithium niobate            quartz/lithium tantalate            lithium tantalate/lithium niobate;                        the two substrates are of the same nature but of different crystal cut. FIG. 2 shows, using the IEEE 1949 conventions in the initial orthonormal coordinate system (X,Y,Z), Z being parallel to the optical axis of the crystal, X being defined by the piezoelectricity of the crystal and Y being perpendicular to (X,Z), the geometrical representation of the cut plane being defined by two successive rotation angles φ and θ. Here, φ corresponds to a first rotation about the Z axis, the coordinate system obtained thus being denoted by (X′,Y′,Z′) with Z′ coincident with Z, and θ corresponds to a second rotation about the X′ axis, the coordinate system obtained thus being denoted by (X″,Y″,Z″), with Z′ coincident with X′. In this final coordinate system, the direction of propagation of the acoustic waves is then defined by a third angle ψ representing a rotation about the Y″ axis. As an example, with these conventions, the values of the angles for the cuts normally used in surface waves for ST quartz are the following:                    φ=0°; θ=42.75°; ψ=0°; and finally                        the two substrates may be of the same nature and same cut. For example, it is possible to use quartz and lithium niobate or lithium tantalate. In this case, as a general rule the assembly operation will be carried out with the same crystal orientation of the two substrates.        
The latter case is particularly interesting in so far as the problems of compatibility between the substrates S1 and S2, in particular the thermal expansion and assembly problems, are implicitly solved. In this case, the orientations of the crystal faces are chosen so as to obtain polarizations of the same direction.
Four major characteristics define the acoustic wave propagation properties of piezoelectric materials. These are:                the velocity V of propagation of the interface acoustic wave. This parameter is important in so far as, for a given operating frequency F, it determines the pitch P of the array of teeth of the electrode combs which is proportional to V/F. When one works at high frequency, typically of the order of a few GHz, achieving high velocities makes it possible to maintain sufficiently wide pitches compatible with current etching technologies;        the attenuation A or insertion loss, expressed in dB per acoustic wavelength, a parameter that it is desired, in general, to reduce so as to reduce the insertion losses of the device;        the first-order coefficient CFT of the variation in frequency as a function of temperature near the ambient temperature, expressed in ppm.C−1. The configuration of the device will be less sensitive to thermal variations the lower this coefficient; and        the electromechanical coupling coefficient k2 representative of the maximum relative frequency band that can be obtained with a device, this coefficient being calculated as the relative half-difference between the velocities of the surface waves on the free substrate and the metalized substrate. In general, this coefficient is the parameter that it is desired to maximize, k2 being a dimensionless parameter expressed as a percent.        
These various parameters, and in particular the coupling coefficient k2, depend strongly on the cut angle of the piezoelectric crystal and on the direction of propagation. It is thus possible to obtain, depending on the cut angle in the case of lithium tantalate, k2 values varying between 0 and 7 as indicated in FIGS. 3a and 3b. FIG. 3a shows the variation of k as a function of (φ,ψ) at zero θ and FIG. 3b shows the variation of k2 as a function of (θ,ψ) at zero φ. In these figures, the discontinuities show the regions within which no possible propagation mode exists. The choice of the cut angle is therefore fundamental. However, the variations in the acoustic characteristics as a function of this angle cannot be determined simply, for example by considerations regarding the crystal structure. They are also very different from those that are obtained in the case of free materials used for the surface acoustic waves.
U.S. Pat. No. 2,799,906 (Pierre Tournois) relating to the production of interface acoustic wave filters gives general recommendations mentioned by way of example and allowing optimum cut angles to be chosen. Particularly mentioned in the case of the use of lithium tantalate that the cuts may be taken along the Y crystallographic axis (where Y is rotated through a certain angle, for example 175°).